1. Field of the Invention
The present invention relates to an apparatus for controlling a speed change ratio of a continuously variable transmission for a vehicle and to a method thereof. Particularly, the invention relates to technology for optimizing a rate of changing the speed.
2. Related Art of the Invention
Methods of controlling a rate of changing the speed of a continuously variable transmission have heretofore been proposed as disclosed in, for example, Japanese Unexamined Patent Publications Nos. 62-149526 and 8-74958.
According to the method of Japanese Unexamined Patent Publication No. 62-149526, a rate of changing the speed is calculated based upon a deviation between a target speed change ratio and an actual speed change ratio.
According to the method of Japanese Unexamined Patent Publication No. 8-74958, the up-shift is discriminated to be an up-shift in a state where the throttle opening degree remains nearly constant and an up-shift accompanying a change in the throttle opening degree, and the up-shift control operation is carried out at different rates of changing the speed.
Even when the deviation between the target speed change ratio and the actual speed change ratio remains the same, however, the speed must be changed at a different rate depending upon whether the deviation is taking place on the side (high-gear side) of a large speed change ratio (in this invention, the speed change ratio=output rotational speed/input rotational speed) or is taking place on the side (low-gear side) of a small speed change ratio. With the constitution in which the rate of changing the speed is set depending simply upon the target speed change ratio and the actual speed change ratio, therefore, it is difficult to change the speed at an optimum rate of changing the speed.
Even in the same up-shift operation accompanying a change in the throttle opening degree, furthermore, the speed must be changed at a different rate depending upon an up-shift of when the throttle valve is brought to a nearly fully closed state from the open state in an attempt to effect the deceleration and an up-shift of when the throttle valve is closed to a half opened state which is smaller than that of during the acceleration in an attempt to shift the acceleration to a steadily traveling state. So far, however, the speed has been changed at the same rate for these up-shifts; i.e., the up-shift operation is not effected at a proper rate of changing the speed.
Similarly, even during the down-shift operation, the speed must be changed at a different rate depending upon a down-shift of when the throttle valve is opened in an attempt to execute acceleration and a down-shift that is effected accompanying a drop in the vehicle speed. So far, however, the down-shift operation has not been effected at such a requested rate of changing the speed.